In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules (such as arsenic, boron or phosphorous) are directly introduced into the wafer to create doped regions, in a process called ion implantation. FIG. 1A illustrates a profile that has been calculated for a typical implant: Arsenic at a nominal energy of 2 keV and dose of 1×1015 atoms/cm2. As shown in FIG. 1A, a depth range of 5.8 angstroms is obtained for a 200 eV energy range, i.e. about 35 eV/angstrom. Note that greater energy results in a deeper distribution. Note also that an increase in dose increases the height of the distribution, if energy is constant. Moreover, an increase in dose increases the depth, if concentration is constant.
Ion implantation normally causes damage to the lattice structure of the wafer. Specifically, in the implantation process, silicon atoms are knocked out of the lattice, and result in vacancies. To remove such lattice damage, the wafer is normally annealed at an elevated temperature, typically 600° C. to 1100° C. Prior to annealing, material properties of the doped regions (such as the concentration of the implants and the depth of the implants) may be measured.
In an implant metrology tool, secondary ion mass spectrometry (SIMS) can be used to determine the profile of the ion implants (i.e. their concentration as a function of depth from the wafer surface). The depth of the profile (at a nominal dose) indicates a measure of the energy used in the ion implantation process. The area under the profile is a measure of the dose of the ion implants. FIG. 1B illustrates a SIMS measurement of concentration as a function of depth from several implanted wafers using the above-described 2 keV and 1×1015 atoms/cm2 process (see paragraph [0001] above). In FIG. 1B, the x axis shows the depth in nanometers and the y axis shows the concentration/cm3 (in atoms/cm2). Note that the deepest and shallowest curves in FIG. 1B are from two wafers that have been processed by a single ion implanter. Since the energy of the ion implanter is better controlled than dose, the energy should be constant, and for this reason, the SIMS process appears to have a depth resolution no better than 1 angstrom, corresponding to ±35 eV of energy.
The SIMS process involves bombarding the wafer with atoms (e.g., oxygen atoms) which collide with atoms on the wafer surface and cause the atoms to be ejected from the surface (i.e., sputtering). During the energy transfer process, a small fraction of the ejected atoms leave as either positively or negatively charged ions which are collected by the mass spectrometer. The ion yield of the wafer is measured and a linear dependence between the ion yield and the doping concentration is used to determine the profile. Since material is sputtered form the processed wafer, causing damage to the sputtered region and re-deposition on nearby regions, the test is destructive and cannot be performed on production wafers during semiconductor wafer fabrication.
Another implant metrology tool uses a four point probe (4PP) arrangement. Specifically, the 4PP arrangement contacts the wafer's surface with four probes arranged in a straight line. Two outer-most probes are used to establish a flow of current through the implanted layer, while the two inner probes are used to measure a voltage drop. The measured voltage is used to deduce the sheet resistance and hence conductivity of the implanted layer and hence the dose. Note that the 4PP arrangement is insensitive to energy used in the ion implantation process. The probes must make contact with the wafer surface, which is considered a destructive process. Also, the measurement needs a large open area of several square millimeters. For these reasons, the 4PP technique is not normally used for process control on production wafers during semiconductor wafer fabrication.
There are several implant metrology tools that measure the damage to the lattice structure in a non-contact manner which is essential to monitor and control the fabrication of wafers. For example, a brochure entitled “TP-500: The next generation ion implant monitor” dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes an implant metrology tool called “TP-500” that requires “no post-implant processing” (column 1, lines 6–7, page 2) and that “measures lattice damage” (column 2, line 32, page 2). The TP-500 includes “[t]wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafers” (columns 1 and 2 on page 2). Such a TW signal is believed to be the result of carrier waves as described in one or more of the following U.S. Patents all of which are incorporated by reference herein in their entirety, namely U.S. Pat. No. 5,042,952 granted to Opsal et al., U.S. Patent.
Damage to the lattice structure can also be evaluated in a non-contact manner by illumination of carriers without creating waves by use of another implant metrology tool called “BX-10” that is available from Applied Materials, Inc., 3050 Bowers Avenue, Santa Clara, Calif. 95054. Carrier illumination is briefly described in U.S. Pat. No. 6,656,749 granted to Paton et al. which is incorporated by reference herein in its entirety. This patent suggests detecting the depth of the source/drain region using a low power laser to excite carriers in the active silicon and a second laser to illuminate the surface. Through use of interferometry, the difference in index of refraction between silicon with excited carriers and silicon with non-excited carriers is to be determined. From the difference in index of refraction, a measurement as to the depth of the activated source/drain regions is to be made. Carrier illumination methods may also be performed as described in each of the following U.S. patents all of which are incorporated by reference herein in their entirety, namely U.S. Pat. No. 6,489,801, U.S. Pat. No. 6,426,644, and U.S. Pat. No. 6,483,594 and U.S. Pat. No. 6,323,951.
The above-described non-contact methods appear (to the inventor of the current invention) to have the following drawback. Specifically, to the extent such methods are based on measuring the damage to the lattice (i.e. the vacancy concentration), the inventor notes that the damage is a function of both dose and energy used during ion implantation. The inventor further notes that higher energy or higher dose, each creates a greater number of vacancies. For this reason, a change in a signal measured by the lattice-damage sensitive methods can indicate a change in either energy or dose (without being able to distinguish therebetween). Hence, an error in dose is easily confused with an error in energy when using a lattice-damage sensitive method.
U.S. Pat. No. 5,862,054 which is incorporated by reference herein in its entirety describes a method to monitor process parameters from multiple process machines to provide real time statistical process control. In this patent, the particular implementation was derived from ion implantation of wafers, but has wide applicability where there are a number of process machines having a number of process parameters and close continuous sampling of data is required. The process parameters are collected on a single computer over a single RS 485 network, and each parameters is analyzed and displayed separately for each process and process machine. Statistical variables like Cp and Cpk arc calculated and presented on the computer screen along with graphs of the various parameters for a particular process machine. Data is aged out of the computer to an archival data base under the control of a manufacturing information system and connected to a company wide network.
U.S. Pat. No. 6,408,220 which is incorporated by reference herein in its entirety describes a a manufacturing environment for a wafer fab, and a statistical process control (SPC) environment for setting control limits and acquiring metrology data of production runs. A computation environment processes the SPC data, which are then analyzed in an analysis environment. A manufacturing execution system (MES) environment evaluates the analysis and automatically executes a process intervention if the process is outside the control limits.